Active damping with a switched capacitor

ABSTRACT

An active damping switching system includes an active damping switching apparatus, including a damping capacitor, a damping resistor coupled to the damping capacitor, an input switch coupled to the damping capacitor, an oscillator coupled to the input switch and configured to open and close the input switch at a frequency, a direct current power source coupled to the active damping switching apparatus, a constant power load and an input filter disposed between the constant power load and the active damping switching apparatus.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of the legally related U.S.Ser. No. 13/218,166 filed Aug. 25, 2011, the contents of which areincorporated by reference herein in their entirety.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to electric power generationand distribution, and more particularly to systems and methods foractive damping with a switched capacitor.

Electrical power systems in hybrid vehicles, such as military hybridvehicles, can include high voltage direct current power generation anddistribution systems. Such electrical systems, however, can experiencestability problems. Constant power loads, such as a switched mode powerconverter may introduce a destabilizing effect on a DC bus, causingsignificant voltage oscillation. The source ripple filter must attenuaterectification ripple and current harmonics resulting from activerectifier switching. The input filter of a switching power convertermust provide forward voltage attenuation of alternating current (AC)voltage superimposed on a DC bus voltage, attenuate current harmonicsresulting from power converter switching and those injected into DC busto allowed levels, and have a low output impedance so as not toadversely affect the stability of switched-mode power converter. A powerconverter's input LC filter without a damper introduces possibleinstability in the presence of constant power (i.e., negative impedance)loads. Traditionally LC or RC damping networks are used to stabilizeunstable loads. The LC damper is connected in series with the inductorof the input LC filter, while an RC damper is connected in parallel withthe capacitor of the input LC filter. The size of damping networks isconsiderably larger than the input LC filter itself, and can thereforeincrease system size and weight.

BRIEF DESCRIPTION OF THE INVENTION

Exemplary embodiments include an active damping switching system,including an active damping switching apparatus, including a dampingcapacitor, a damping resistor coupled to the damping capacitor, an inputswitch coupled to the damping capacitor, an oscillator coupled to theinput switch and configured to open and close the input switch at afrequency, a direct current power source coupled to the active dampingswitching apparatus, a constant power load and an input filter disposedbetween the constant power load and the active damping switchingapparatus.

Additional exemplary embodiments include an active damping switchingsystem, including an active damping switching apparatus, including adamping capacitor, a damping resistor coupled to the damping capacitor,an input switch coupled to the damping capacitor, a zero cross detectorcoupled to the input switch, a direct current power source coupled tothe active damping switching apparatus, a constant power load and aninput filter disposed between the constant power load and the activedamping switching apparatus.

Additional exemplary embodiments include an active damping switchingsystem, including an active damping switching apparatus, including afirst input switch, a first damping resistor coupled to the first inputswitch, a second input switch, a damping capacitor coupled to the secondinput switch, a second damping resistor coupled to the dampingcapacitor, a first gate drive coupled to the first input switch and asecond gate drive coupled to the second input switch an to the firstgate drive, a direct current power source coupled to the active dampingswitching apparatus, a constant power load and an input filter disposedbetween the constant power load and the active damping switchingapparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 illustrates a DC electric power system with a switched capacitorstabilization network with an external oscillator;

FIG. 2 illustrates DC electric power system with a switched capacitorstabilization network with a combination of external oscillator andfilter capacitor feedback current;

FIG. 3 illustrates a DC electric power system with a switched capacitorstabilization; and

FIG. 4 illustrates DC electric power system with dual control activedamping.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary embodiments include systems and methods for active damping byimplementing a switched RC stabilization network. In response to currentripple on the DC bus, the RC stabilization network can be switched on todampen the ripple. FIG. 1 illustrates an active damping system 100. Inone embodiment, the system 100 can include a DC power source 105electrically coupled to a constant power load 120 via input filter 115.In one embodiment, the DC source 105 can be an AC generator rectified toa DC voltage that includes voltage/current ripples. The system 100 canfurther include a switched capacitor stabilization network 110. In oneembodiment, the input filter 115 is an LC filter having an inputcapacitor 125 and an input inductor 130 having values selected to filterout certain frequencies in voltage/current that exists between the dcpower source 105 and the constant power load 120.

In one embodiment, the switched capacitor stabilization network 110includes an input switch 135 electrically coupled to the dampingcapacitor 140. The damping capacitor 140 is coupled to a dampingresistor 145 that is coupled to a point between the input inductor 130and the input capacitor 125 of the input filter 115. The switchedcapacitor stabilization network 110 can further include an oscillator150 tuned at frequency equal or close to the resonant frequency of theLC input filter 115

$\left( {f = \frac{1}{2\; \pi \sqrt{LC}}} \right).$

In one embodiment, if the system 100 becomes unstable, the switchedcapacitor stabilization network 110 stabilizes the system 100 by closingswitch 135 which places the damping capacitor 140 and the dampingresistor 145 in parallel with the input capacitor 125 of the inputfilter 115 for a period of time, by directly controlling the inputswitch 135. In one embodiment, the oscillator 150 switches the dampingcapacitor 140 and the damping resistor 145 into parallel with the inputcapacitor 125 at a frequency approximately equal to the resonantfrequency of the LC input filter 115. By matching the frequency of theoscillator 150 to the resonant frequency of the input filter 115, systemoscillations can be eliminated.

FIG. 2 illustrates an active damping system 200. In one embodiment, thesystem 200 can include a DC power source 205 electrically coupled to aconstant power load 220 via input filter 215. The system 200 can furtherinclude the switched capacitor stabilization network 210. In oneembodiment, the input filter 215 can be an LC filter having an inputcapacitor 225 and an input inductor 230 tuned to filter out certainfrequencies in voltage/current that exists between the dc power source205 and the constant power load 220. The input filter 215 furtherincludes a current sensor 275 coupled to the input capacitor 225 asfurther described herein.

In one embodiment, the switched capacitor stabilization network 210includes an input switch 235 electrically coupled to the return path ofDC power source 205. The input switch 235 is coupled to a dampingcapacitor 240. The damping capacitor 240 is coupled to a dampingresistor 245 that is coupled to a point between the input inductor 230and the input capacitor 225 of the input filter 215. The switchedcapacitor stabilization network 210 can further include an oscillator250 tuned at frequency equal or close to the resonant frequency of theLC input filter 215

$\left( {f = \frac{1}{2\; \pi \sqrt{LC}}} \right).$

In one embodiment, if the system 200 becomes unstable, the switchedcapacitor stabilization network 210 stabilizes the system 200 by closingswitch 235 which places the damping capacitor 240 and the dampingresistor 245 in parallel with the input capacitor 225 of the inputfilter 215 for a period of time, by directly controlling the inputswitch 235. In one embodiment, the oscillator 250 switches the dampingcapacitor 240 and the damping resistor 245 into parallel with the inputcapacitor 225 at a frequency approximately equal to the resonantfrequency of the LC input filter 215. By matching the frequency of theoscillator 250 to the resonant frequency of the input filter 215, systemoscillations can be eliminated.

In one embodiment, the switched capacitor stabilization network 210 canfurther include filter capacitor feedback current to control the dampingin the system 200. As such, the switched capacitor stabilization network210 can further include a transition conditions controller 255 coupledto the oscillator 250 via a second switch 260. In one embodiment, thetransition conditions controller 255 can switch between the dampingprovided by the oscillator 250 and damping provided by the filtercapacitor feedback current. The switched capacitor stabilization network210 can therefore further includes a low pass filter 265 to selectfrequency due to system instability. The filter 265 is coupled to a zerocross detector 270. Transition condition block 255 controls switch 260in response to the magnitude of the current ripple detected by thecurrent sensor 275. During initial detection of unstable system 200operation, current magnitude is above pre-determined level and switch260 is connected to the output of zero-cross detector 270 forcingstabilization switch 235 to respond to the frequency detected by thecurrent sensor 275. When current magnitude falls below pre-determinedlevel, the transition condition block 255 reconnects control input ofswitch 235 to the output of the oscillator 250 via switch 260. Thisapproach benefits system stabilization by providing fast reduction ofvoltage/current ripple during initial detection of system instabilityand reduction of voltage/current ripple during steady-state operation.The filter capacitor feedback current from the input capacitor 225 inthe input filter 215 is detected by a current sensor 275 coupled betweenthe input capacitor 225 and the filter 265, and disposed in the inputfilter 215. In one embodiment, the current sensor 275 can detect thefilter capacitor feedback current from the input capacitor 225 and passthe current to the filter 265.

The transition conditions controller 255 can be any suitablemicrocontroller or microprocessor for executing the instructions (e.g.,on/off commands) described herein. As such, the suitable microcontrolleror microprocessor can be any custom made or commercially availableprocessor, a central processing unit (CPU), an auxiliary processor amongseveral processors, a semiconductor based microprocessor (in the form ofa microchip or chip set), a microprocessor, or generally any device forexecuting software instructions.

In another embodiment, active damping is also attained by synchronizingswitch control with the frequency due to system instability. FIG. 3illustrates an active damping system 300. In one embodiment, the system300 can include a DC power source 305 electrically coupled to a constantpower load 320 via input filter 315. In one embodiment, the DC source305 can be an AC generator which output is rectified to a DC voltagethat includes voltage ripple. The system 300 can further include theswitched capacitor stabilization network 310. In one embodiment, theinput filter 315 can be an LC filter having an input capacitor 325 andan input inductor 330 having values selected to filter out certainfrequencies between the switched capacitor stabilization network 310 andthe constant power load 320. In one embodiment, the switched capacitorstabilization network 310 further includes an input switch 335electrically coupled to a damping capacitor 340, which is furthercoupled to a damping resistor 345, which is coupled to the input filter315. The switched capacitor stabilization network 310 can furtherinclude filter capacitor feedback current to control the damping in thesystem 300. The switched capacitor stabilization network 310 cantherefore further include a low pass filter 365 to select frequency dueto system instability. The filter 365 is coupled to a zero crossdetector 370. In one embodiment, the zero cross detector 370 isimplemented such that the input switch 335 is turned on/off as thefilter capacitor feedback current crosses zero. The input filter 315further includes a current sensor 375 coupled between the inputcapacitor 325 and the filter 365. The filter capacitor feedback currentfrom the input capacitor 325 in the input filter 315 is detected by thecurrent sensor 375. In one embodiment, the current sensor 375 can detectthe filter capacitor feedback current from the input capacitor 325 andpass the current to the low pass filter 365. When the zero crossdetector 370 detects the filter capacitor feedback current crossingzero, the input switch 335 is closed placing the damping capacitor 340and the damping resistor 345 in parallel with the filter capacitor 325thereby providing active damping based on the filter capacitor feedbackcurrent.

In another embodiment active damping can also be attained bysynchronizing multiple switches within a switched capacitorstabilization network for dual control. FIG. 4 illustrates an activedamping system 400. In one embodiment, the system 400 can include a DCpower source 405 electrically coupled to a constant power load 420 viainput filter 415. In one embodiment, the DC source 405 can be an ACgenerator which output is rectified to a DC voltage that includesvoltage ripple. The system 400 can further include the switchedcapacitor stabilization network 410, coupled to a constant power load420. In one embodiment, the input filter 415 can be an LC filter havingan input capacitor 425 and an input inductor 430 having values selectedto filter out certain frequencies between the dc power source 405 andthe constant power load 420. The input filter 415 further includes acurrent sensor 475 described further herein.

The system 400 includes a first input switch 434 coupled to the DC powersource 405 and a first damping resistor 444. The first damping resistor444 is further coupled to the input filter 415. The system 400 furtherincludes a second input switch 435 coupled to the damping capacitor 440and to a second damping resistor 445, which is coupled to the inputfilter 415. Each of the first and second switches 434, 435 isrespectively coupled to a first and second gate drive 480, 481. Thefirst gate drive 480 is coupled to an “and” function 472 and a firstzero cross detector 470. The second gate drive 481 is coupled to asecond zero cross detector 471. As described further herein, the firstgate drive 480 activates the first switch 434 when current inputs toboth the first and second zero cross detectors 470, 471 pass zero, andthe second gate drive 481 activates the second switch 435 when currentinput into the second zero cross detector 471 passes zero. Dual controlof stabilization network 410 via switch 434 and switch 435 allows sizereduction of damping capacitor 440 and minimize power losses in dampingresistor 444. The second zero cross detector 471 is also coupled to aband pass filter 463 that is coupled to the current sensor 475 in theinput filter 415. The band pass filter 463 is coupled to an absolutevalue unit 464, which is coupled to a low pass filter 465, which filtersour any undesirable high frequencies, such as high frequency harmonics.The band pass filter 463 is tuned to the frequency defined by the inputfilter 415

$\left( {f = \frac{1}{2\; \pi \sqrt{LC}}} \right).$

The absolute value unit 464 provides a positive number for latercomparison to a current ripple reference 466. The low pass filter 465and the current ripple reference 466 are coupled to a comparator 467that is coupled to the first zero cross detector 470. Current is inputinto the first zero cross detector 470 if the current from the low passfilter 465 exceeds the current ripple reference 466.

On one embodiment, the system 400 attains active damping by closing andsynchronizing the input switches 434, 435. Technical effects include theimprovement of power quality of the system bus by providing activedamping. In addition, the systems and methods described herein reducesystem weight, size, and cost by reducing damping capacitor byapproximately three times in comparison the capacitor size requiredusing passive techniques.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

1. An active damping switching system, comprising: an active damping switching apparatus, including: a damping capacitor; a damping resistor coupled to the damping capacitor; an input switch coupled to the damping capacitor; a zero cross detector coupled to the input switch; a direct current (DC) power source coupled to the active damping switching apparatus; a constant power load; and an input filter disposed between the constant power load and the active damping switching apparatus.
 2. The system as claimed in claim 1 wherein the zero cross detector is configured to switch the input switch on and off responsive to an input filter current and to split the current from the input filter in response to transient inputs to the damping capacitor.
 3. The system as claimed in claim 1 wherein the input filter is an LC filter comprising an input inductor coupled to the DC power source and an input capacitor coupled to the input inductor.
 4. The system as claimed in claim 3 further comprising: a current sensor coupled to the input capacitor; and a low pass filter coupled between the current sensor and the zero cross detector.
 5. An active damping switching system, comprising: an active damping switching apparatus, including: a first input switch; a first damping resistor coupled to the first input switch; a second input switch; a damping capacitor coupled to the second input switch; a second damping resistor coupled to the damping capacitor; a first gate drive coupled to the first input switch; and a second gate drive coupled to the second input switch and to the first gate drive; a direct current (DC) power source coupled to the active damping switching apparatus; a constant power load; and an input filter disposed between the constant power load and the active damping switching apparatus.
 6. The system as claimed in claim 5 wherein the first gate drive is coupled to the second gate drive via an “and” unit.
 7. The system as claimed in claim 5 wherein the input filter is an LC filter comprising an input inductor coupled to the DC power source and an input capacitor coupled to the input inductor.
 8. The system as claimed in claim 7 further comprising a current sensor coupled to the input capacitor; and a band pass filter coupled between the current sensor and the zero cross detector.
 9. The system as claimed in claim 8 further comprising: a first zero cross detector coupled to the first gate drive; a low pass filter coupled to the first zero cross detector; and an absolute value unit coupled to the low pass filter and to the band pass filter.
 10. The system as claimed in claim 8 further comprising a second zero cross detector coupled to the second gate drive and to the band pass filter. 